As the presidential campaigns shift from televised conventions to the general election, a new danger to the electoral process may well emerge in the form of highly targeted online advertising unless the Federal Election Committee makes such advertising visible to all.

Imagine that one of your neighbors keeps seeing campaign ads sharply attacking current environmental regulations, another sees ads passionately arguing for stronger environmental regulations and the campaign ads you see never mention the environment.

Now imagine that all these ads come from the same candidate.

This can’t happen on TV — but it’s business as usual on the Internet. As you browse the Web, many ads that appear on your screen were selected just for you. A merchant may already offer your neighbor a lower price than you see for a new camera, so why not different campaign promises too?

There’s nothing new about politicians trying to be all things to all people. But new technology gives them an unprecedented ability to do so. It’s as if a politician could secretly whisper a personalized message to every voter. We choose our leaders and the direction of the country based on the messages we get from candidates, so we must be able to believe these messages.

With TV or newspaper ads, we all see the same message, and those who disagree can start a healthy debate that benefits us all. Online ads, in contrast, can be narrowly targeted.

Do you browse Huffington Post or FoxNews.com? Did you read articles about same-sex marriage or immigration? Do you care so much about these issues that they appear in your personal email and Google searches? Have you searched online for a job, a new rifle or contraception information? These are hints about your views on extending unemployment insurance, restricting gun sales and requiring health insurance plans to cover birth control. Did you access the Internet from Beverly Hills or Harlem?

Political campaigns can use all of this to understand which issues matter to you and where you stand better than your own family realizes. Campaigns can then quietly tell you what you want to hear.

One problem is that a politician can now nimbly play both sides of an issue, perhaps telling some voters how he dislikes some aspects of the Obama health care plan, and others how he would protect the plan from being overturned.

Another is that narrowly targeted ads with questionable claims may escape a fast rebuttal. During the 2000 South Caroline primary, an independent group, running a push poll, telephoned voters one by one and spread the false rumor that Sen. John McCain (R-Ariz.) had fathered a black child out of wedlock. Today, they’d use targeted online ads.

Probably the most serious problem is that targeted ads may persuade us each to vote based on a different issue — so there is no shared view of which issues matter. Leaders can make serious change on only a few issues at a time. Candidates usually tell us which issues they consider priorities, and getting elected gives them political capital to act on those issues. But if citizens are influenced by customized ads, we won’t know which issues matter to candidates. More important, they’ll have no popular mandate when it’s time to govern.

Besides, if a targeted ad said one mayoral candidate agrees with you on the issue you care most about, but the candidate doesn’t see that issue as important, perhaps you should vote for her opponent.

“Sunlight,” Supreme Court Justice Louis Brandeis wrote, “is the best disinfectant.” The Federal Election Commission already oversees paid ads from candidates and political committees to ensure that the public knows who paid for them. The FEC should also require that online ads be placed on publicly available websites. Then, a politician who runs targeted ads that appear to take every side of every conceivable issue can more easily be held accountable.

Meanwhile, President Barack Obama and GOP presidential nominee Mitt Romney should lead by example and post all their online campaign ads voluntarily.

Jon Peha, a professor at Carnegie Mellon University, is a former chief technologist of the Federal Communications Commission.